1. Field of the Invention
The present invention generally relates to a data reproducing method, a data reproducing device and a magneto-optical disk device, more particularly, to a data reproducing method, a data reproducing device and a magneto-optical disk device which use a partial-response/maximum-likelihood (PRML) detecting method.
2. Description of the Related Art
For example, a magneto-optical disk device has been implemented into various fields such as in recording and reproduction of picture/image information and in recording and reproduction of various sorts of code data for computers, due to its high capacity, low cost and high reliability. Such a magneto-optical disk device is expected to have an even higher capacity; accordingly, this requires a data reproducing method that can with high accuracy reproduce data recorded with high density.
There is a method for reproducing data with high accuracy including the processes of recording data in a magneto-optical disk by modulating the data into a so-called partial response (PR) waveform, sampling signals reproduced from the magneto-optical disk at a predetermined interval, and thereafter detecting most likely data by a so-called Viterbi detector (a maximum-likelihood data detector).
For example, a data reproducing system of a magneto-optical disk device has enhanced its reproducing capability by calculating a fluctuation amount (hereinafter referrer to as an offset amount) of direct current components of reproduction signals of data being reproduced from a magneto-optical disk, and feeding back the calculated offset amount to an expected value of a PRML reproducing system.
FIG. 1 shows an example of a structure of a data reproducing system 100. In FIG. 1, an analog-to-digital converter 101 is supplied with an analog reproduction signal reproduced from a magneto-optical disk, for example, and then converts the analog reproduction signal into a digital signal. A digital equalizer 102 shapes the waveforms of the digital signal supplied from the analog-to-digital converter 101, and then supplies sampled values to a Viterbi detector (a maximum-likelihood detector) 103.
The Viterbi detector 103 detects recorded data from the sampled values of the analog reproduction signal supplied from the analog-to-digital converter 101 via the digital equalizer 102 according to a Viterbi decoding algorithm.
The sampled values of the analog reproduction signal are supplied to a branch-metric calculation unit (hereinafter referred to as a BM) 104 of the Viterbi detector 103. The BM 104 calculates a branch-metric value (hereinafter referred to as a BM value) that is a difference between each of sampled values yt supplied thereto and an expected value. The expected value is a value depending on a PR waveform, and is a value that the analog reproduction signal could essentially assume. The BM value is calculated for each expected value when the sampled value yt is supplied.
An add-compare-select unit (hereinafter referred to as an ACS) 105 adds each of the above-mentioned BM values to a pass-metric value (hereinafter referred to as a PM value) of one clock before that is stored in a pass-metric memory (hereinafter referred to as a PMM) 106, and compares every two PM values after the addition. Then, the ACS 105 selects the smaller of the every two PM values as a new PM value according to the comparison result, and then stores the selected PM value in the PMM 106. Selecting the smaller PM value in this way means selecting a state-transition pass. That is, the ACS 105 always selects a state-transition pass with the minimum PM value.
A pass memory (hereinafter referred to as a PM) 107 is supplied from the ACS 105 with data (binary data) corresponding to the passes selected as described above. The PM 107 shifts the data corresponding to each of the selected passes one by one, and in this course, weeds out data corresponding to each of unselected passes one by one. Then, the PM 107 outputs data corresponding to the surviving pass as a demodulated signal.
On the other hand, the ACS 105 supplies the selected PM values to a minimum-value selector 108. The minimum-value selector 108 selects the minimum PM value, and then supplies the selected PM value to an offset-amount detector 109. The offset-amount detector 109 calculates an offset amount according to the supplied PM value by using a sliding average method, for example. The data reproducing system 100 shown in FIG. 1 feeds back the calculated offset amount to the expected value of a PRML reproducing system by adding the calculated offset amount to the expected value and supplying the expected value including the offset amount to the BM 104.
FIG. 2 shows another example of a structure of the data reproducing system 100. In FIG. 2, the digital equalizer 102 shapes the waveforms of the digital signal supplied from the analog-to-digital converter 101, and then supplies sampled values to the Viterbi detector 103 and a comparator 111. The comparator 111 is supplied with not only the sampled values of the analog reproduction signal but also a total value of an offset amount from the offset-amount detector 109 and a threshold value.
The comparator 111 compares the sampled values supplied thereto with the total value of the offset amount and the threshold value, and then supplies the comparison results to a state detector 112. The state detector 112 judges the state of the sampled values on the basis of the comparison results supplied thereto, and then supplies the judgment results to the offset-amount detector 109 and a respective-expected-value calculator 113.
The respective-expected-value calculator 113 feeds back the offset amount to the expected value of a PRML reproducing system by calculating respective expected values according to the supplied judgment results and supplying the calculated respective expected values to the BM 104. It is noted that the offset-amount detector 109 calculates the offset amount according to the judgment results supplied from the state detector 112.
FIG. 3 shows still another example of a structure of the data reproducing system 100. In FIG. 3, the digital equalizer 102 shapes the waveforms of the digital signal supplied from the analog-to-digital converter 101, and then supplies sampled values to the Viterbi detector 103 and a shift register 114. The shift register 114 delays the sampled values of the analog reproduction signal by a predetermined time, and supplies the delayed to one terminal of an AND circuit 116.
On the other hand, the PM 107 outputs data corresponding to the surviving pass as a demodulated signal, and also supplies the demodulated signal to a state detector 115. The state detector 115 judges the state of the sampled values on the basis of the demodulated signal supplied thereto, and then supplies the judgment results to the other terminal of the AND circuit 116.
The AND circuit 116 calculates logical products of the sampled values supplied from the shift register 114 and the judgment results supplied from the state detector 115, and then supplies the calculation results to the respective-expected-value calculator 113. The respective-expected-value calculator 113 feeds back an offset amount to an expected value of a PRML reproducing system by calculating respective expected values according to the supplied calculation results and supplying the calculated respective expected values to the BM 104.
However, the data reproducing system 100 shown in FIG. 1 cannot correctly select the minimum PM value when the difference between a sampled value and an expected value is large. Therefore, the data reproducing system 100 shown in FIG. 1 problematically miscalculates an offset amount in some cases.
There is also a problem that the data reproducing system 100 shown in FIG. 2 has to have an augmented circuit scale for comparing a threshold value with a sampled value. Additionally, the data reproducing system 100 shown in FIG. 2 involves a problem that it is difficult to determine a threshold value since the threshold value itself is required to follow an offset amount.
The data reproducing system 100 shown in FIG. 3 suffers a problem that utilizing the demodulated signal output from the PM 107 entails a delay corresponding to the time required to perform the process in the PM 107 so as to delay the feedback to the expected value. In addition, since the data reproducing system 100 shown in FIG. 3 uses the demodulated signal output from the PM 107, the data reproducing system 100 requires the shift register 114 for delaying the sampled values. Therefore, there is also a problem that the data reproducing system 100 shown in FIG. 3 has to have an enlarged circuit scale.